Supramolecular features in the engineering of 3d metal complexes with phenyl-substituted imidazoles as ligands: the case of copper(ii)

Abstract

With the aim of recording and assessing the role and impact of various non-covalent interactions activated during the self-assembly process of 3d metal complexes with organic ligands, the synthesis and X-ray characterization of a series of eleven Cu(II) complexes with two phenyl-substituted imidazoles [1-methyl-4,5-diphenylimidazole (L) and 4,5-diphenylimidazole (HL′)] as ligands have been carried out. A variety of parameters and conditions have been probed using the general Cu^II/X⁻/L or HL′ (X⁻ = Cl⁻, Br⁻, I⁻, NO₃⁻, NO₂⁻ or ClO₄⁻) reaction system. In structures with the ligand L (1–7), lacking any group capable of establishing strong intermolecular H-bonding contacts, the burden of the supramolecular organization is undertaken by weak, yet productive, C–H⋯X and C–H⋯π interactions, while the metal ions keep their preferred coordination geometries (square planar and square pyramidal). In compounds with the ligand HL′ (8–11), however, robust and recurring N–H⋯X (X = Cl or O) motifs have the leading role towards effectively directing the self-assembly; flexible water molecules contribute actively, when needed {[Cu(HL′)₄](ClO₄)₂·EtOH·CH₂Cl₂·H₂O}, to maximize the number of these motifs. At a second level of organization, additional subordinate C–H⋯Cl or O and C–H⋯π interactions complement the rigidity of the structures. The distortion of the coordination environment (seesaw) of Cu^II centres in compounds [CuCl₂(HL′)₂]·Me₂CO·0.25H₂O and [CuCl(HL′)₃]Cl·0.6H₂O appears to reflect the balance of forces within the crystallization system to facilitate the necessary orientation of the tectons so as to form the hierarchically prevailing N–H⋯X synthons. Moreover, the small number of intermolecular π⋯π contacts observed, despite the abundance of aromatic rings, indicates their weakness to form at the expense of stronger interactions. Lastly, comparison with previously structurally characterized analogous Co(II), Ni(II) and Zn(II) complexes (vide infra) corroborates and adds confidence to the conclusions deduced herein.